The present invention relates to an image forming apparatus that has a function to avoid image shift even when a sheet conveyance unit is drawn out of an apparatus main body for jam clearance or for apparatus maintenance and is mounted again, and is capable of being applied to a printer (a printing machine), a copying machine and a multifunction peripheral having functions of the printer and the copying machine.
In recent years, it has become popular to use image forming apparatuses such as a printer, a copying machine and a multifunction peripheral having functions of the printer and the copying machine wherein shift of the sheet to be fed to an image forming unit is detected, and an amount of shift of the sheet is fed back to the image forming unit (an image forming system) to correct a position to start writing images in the image forming system.
In relation to an image forming apparatus having a function to correct the shift of a sheet of this kind in an image forming system, there is disclosed an image forming apparatus in Unexamined Japanese Patent Application Publication No. 08-059069. This image forming apparatus detects a sheet conveyance position for a transfer sheet with a shift detection sensor, and is equipped with a memory that is capable of rewriting when storing plural sheet conveyance position data corresponding to sheet sizes, and it is one wherein sheet conveyance position data in the memory can be rewritten by key input on an operation section. When an apparatus is constructed in the aforesaid manner, a reference position of a transfer sheet in the case of sheet conveyance can be changed easily, concerning errors of mounting for a sheet conveyance unit that holds the shift detection sensor.
Further, in relation to a technology to correct an image forming position, Unexamined Japanese Patent Application Publication No. 2002-014500 discloses an image forming apparatus. In this image forming apparatus, there are provided an image carrier, a sheet conveyance device, a sheet edge detection device and a writing timing correction device, and the image carrier is given exposure, and a latent image is formed on the image carrier. The sheet conveyance device conveys a sheet to the position onto which a visible image formed on the image carrier is conveyed. A sheet edge detection device detects an edge portion of the sheet.
The writing timing correction device inputs results of detection by the sheet edge detection device to correct writing timing in the main scanning direction for the latent image formed on the image carrier. The main scanning direction is a direction that is nearly perpendicular to a conveyance direction (a sub-scanning direction) for sheets. Under the assumption of the foregoing, the sheet edge detection device detects both edges of the sheets to be conveyed. When an apparatus is constructed in the aforesaid manner, the precision in the case of lateral registration operations can be improved, which makes it possible to cope with precisions required by users flexibly.
Further, in relation to a technology to adjust registrations in the main scanning direction and sub-scanning direction, Unexamined Japanese Patent Application Publication No. 2004-233712 discloses a sheet conveyance apparatus. In this sheet conveyance apparatus, for conveying a recording sheet to an image forming section that conducts image forming on the recording sheet through a sheet conveyance path, there are provided an irradiation device, a light-receiving device and a control device, and the irradiation device is arranged on the sheet conveyance path to irradiate light that extends in the lateral direction of the recording sheet perpendicular to the conveyance direction for the recording sheet on the sheet conveyance path.
The light-receiving device is arranged to face the irradiation device with a sheet conveyance path interposed between, to receive light irradiated from the irradiation device and thereby to send out light-receiving signals corresponding to the amount of light-receiving. Under the assumption of the foregoing, the control device detects shift in lateral registration in the lateral direction, corresponding to an amount of light-receiving, and adjust image forming timing of the image forming section. Together with this, the control device detects shift of longitudinal registration in the conveyance direction for recording sheets, corresponding to the maximum amount of light-receiving an amount of a light-receiving device whose control device prescribed in advance, to control timing for start writing images of the image forming section.
When constituting in the aforesaid manner, it is possible to adjust longitudinal registration substantially for each recording sheet with a high degree of accuracy, and thereby to prescribe an image position on a recording sheet accurately.
However, an image forming apparatus wherein shift of sheet is detected, then, an amount of the shift is fed back to an image forming system and a position to start writing images is corrected, has following problems.
i. In the image forming apparatus shown in Unexamined Japanese Patent Application Publication No. 08-059069 and in Unexamined Japanese Patent Application Publication No. 2002-014500, a sheet conveyance unit (a sheet conveyance device) including a shift detection sensor (sheet reference position) is of the structure enabling the unit to be drawn out of the apparatus main body to be opened, for making JAM clearance to be easy. The JAM clearance means operations to remove sheets jammed in the image forming section for some reasons to be reluctant.
When a sheet conveyance unit including a shift detection sensor is mounted on or dismounted from the apparatus main body for JAM clearance, a position of the sheet conveyance unit for the apparatus main body is varied, and there is sometimes an occasion wherein positional shift between the adjusted conveyance reference position and a sheet conveyance unit mounting position after the JAM clearance is caused. This positional shift is sometimes fed back to the position for start writing images in the image forming system (which is also called an image forming section hereafter) to become a cause for image forming positional shift.
ii. Also in the case of mounting on and dismounting from for the sheet conveyance unit and of resetting for maintenance or the like, there is sometimes an occasion where the positional shift for the image is caused in the same way. It has been confirmed that the positional shift of an image is caused by relative positional shift between a writing unit and a shift detection sensor (which is also called a sheet detection section hereafter) caused by errors and dispersion of mounting a sheet conveyance unit. Therefore, operations of adjusting a conveyance reference position of a sheet conveyance unit for the apparatus main body are needed, each time.
iii. Even in the case where the sheet conveyance apparatus shown in Unexamined Japanese Patent Application Publication No. 2004-233712 is mounted, the problems mentioned above are still caused by the shift between the apparatus main body and the sheet conveyance unit and by the shift between the apparatus main body and the sheet conveyance unit in terms of conveyance reference position which are caused by the strain of the apparatus, generating the same image positional shift, which are caused by vibration in the case of moving the apparatus and by a difference of environment for installation of the apparatus.
Therefore, an object of the invention is to provide an image forming apparatus wherein the aforesaid problems are solved, the relative positional relationship between the image forming section and the sheet detection section can be maintained to be constant, and a position for writing images in an image forming section can be corrected more precisely in comparison with a conventional method.